For example, in portable small and thin electronic devices (mobile devices) such as a smartphone and a tablet terminal, sheet-like heat conducting members such as a metal plate and a heat diffusion sheet are widely used to transfer heat of a heat generating component. For example, as the metal plate, copper, aluminum, a magnesium alloy, a thin plate formed by stacking the copper, the aluminum, and the magnesium alloy, and the like are used. The performance of heat conduction of the metal plate depends on the thermal conductivity of a material. For example, as the heat diffusion sheet, a graphite sheet is used. The thermal conductivity of the graphite sheet is approximately 500 to 1500 W/mK. At the thermal conductivity of this degree, when the amount of heat of the heat generating component increases, it is likely that heat transfer is unable to be sufficiently performed only by heat conduction of the material.
Therefore, in order to efficiently transfer and diffuse a larger heat quantity, it is conceivable to use a heat pipe, which is a heat transfer device that makes use of the latent heat of a vaporized fluid. For example, when heat transfer of a heat pipe having a diameter of approximately 3 to 4 mm is converted into thermal conductivity, the thermal conductivity is equivalent to approximately 1500 to 2500 W/mK. The heat pipe illustrates a large value compared with a sheet-like heat conducting member that makes use of heat conduction of a material.
However, for efficient heat transport, the diameter of a pipe, which is a heat transfer pipe, requires to be increased in order to improve fluidity of the working fluid. This is a problem in mounting the heat pipe on an electronic device, therefore the application of the heat pipe to a mobile device does not make a progress. In this case, it conceivable to form the heat pipe flat in shape. However, when the heat pipe is formed flat, a flow of a working fluid in the pipe is interrupted, and the performance of the heat transport is deteriorated.
On the other hand, in a loop heat pipe, flows of a vapor-phase working fluid and a liquid-phase working fluid are independent from each other, and flowing direction of the working fluid is one direction. Compared with the heat pipe in which the liquid-phase working fluid and the vapor-phase working fluid are opposed to each other in a single pipe, the flow resistance of the working fluid in a loop heat pipe may be reduced, and the heat transport will be possible to be efficient. Therefore, it is conceivable to use the loop heat pipe for a mobile device.
Related art is described in, for example, Japanese Laid-open Patent Publication No. 60-178291 and International Publication Pamphlet No. WO 2015/087451.